two keys to changing habits

One reason why so many of us fail with habit change is that we try to do too much at once and don't do what we are trying to do for long enough.
I heard today something that I thought was helpful. The person was saying that it takes doing something 18 times over a minumum of 18 days to create a new habit.
Others I've read have suggested that for most of us we can only create one new habit every month. (It's too mentally exhausting to try and change too much at once and we end up failing in all our new efforts.)
Key 1: Concentrate on no more than one new habit per month.
So try this experiment. Pick a new habit, just one, and then repeat that one habit 18 times over at least the next 18 days. For example, if the habit is going for a walk and you do it every 2 days, it will take 18 times over the next 36 days to create that new habit. If the habit is something you only do once a week then it will take 18 weeks to create the new habit, and in this case you can add a second habit at the end of the first month.
Key 2: Add new habits rather than stopping old habits.
And here's another suggestion I've found helpful. I've found it easier to add new habits rather than stop old habits. (Though some things still need to be stopped immediately.) So for example I'm wanting to cut down on the caffiene I drink, so instead of stopping drinking coffee I've added a regime of drinking a glass of water every time I have a coffee. The new good habit has the tendency of pushing out the old bad habit without all the pain normally associated with trying to give up something.
All the best on your habit changes.